This invention relates, in general, to handling semiconductor wafers during wafer processing, and more particularly to mounting a semiconductor wafer onto a submount to reduce unsupported semiconductor wafer handling during wafer processing.
A semiconductor chip operated at extremely high speeds tends to dissipate large amounts of heat. If large power devices are present on the semiconductor chip, it is essential to operate them at a temperature as low as possible. Lowering the operating temperature of the semiconductor chip will benefit operation by increasing frequency response due to higher carrier mobility and reduced parasitic resistances. It will also increase reliability and reduce the chance of a semiconductor chip failing in the field by reducing heat stress.
Gallium arsenide and other compound semiconductors are poor thermal conductors. To maximize heat transfer, a gallium arsenide semiconductor chip is made as thin as possible. A heat sink can be coupled to the semiconductor chip to remove heat from the semiconductor chip.
Gallium arsenide wafers used for fabricating semiconductor chips are extremely brittle and prone to breakage when handled during wafer processing. This fact is further complicated when a gallium arsenide wafer is thinned to increase thermal transfer through the wafer. How the wafer is handled during a wafer thinning process, controlling uniformity of wafer thickness, handling the wafer after it has been thinned, and further processing through high temperature back-metal deposition are only a few of the problems which must be dealt with in gallium arsenide wafer processing.
Solutions for these problems revolve around mounting the gallium arsenide wafer to a submount. The submount is used to handle the wafer during wafer process steps and to provide support or rigidity to the gallium arsenide wafer. This increases processing yields by preventing gallium arsenide wafer fracturing. An adhesive material or bonding material is needed to bond the gallium arsenide wafer to the submount.
The submount is typically made of glass, quartz, sapphire, metal, alumina, gallium arsenide, or silicon. Some of these submount materials will induce stress to the gallium arsenide wafer when bonded together due to different material thermal expansion rates. Temperature changes cause the submount, bonding material, and the gallium arsenide wafer to expand or contract by different amounts. The different rates of expansion or contraction induce stress on the gallium arsenide wafer causing it to bow or warp. Grinding a warped gallium arsenide wafer to reduce wafer thickness under these conditions will not yield a wafer having a uniform thickness.
There are a number of adhesives commonly used to bond semiconductor wafers to a submount. The bond formed, however, must now be strong enough to withstand the grinding process, plus not liquify, sublimate, or wash away during subsequent wafer process steps. Also, the bond must be easily broken to remove a thinned gallium arsenide wafer after the processing steps have been completed.
Wax is a very good choice for bonding material in a production environment because it is simple to apply and control. Two major drawbacks in using wax are developing a methodology for applying the wax in a production environment and the fact that wax has a low liquifying point which cannot withstand high temperature wafer process steps. Ideally, a wax bond between the gallium arsenide wafer and submount must have a constant or uniform thickness throughout the bond to minimize error when reducing wafer thickness.
Wax application to form a bond is very crude and does not lend itself to a production environment. Wax shavings are melted in a crucible. The melted wax is poured onto the submount. The gallium arsenide wafer and the submount are pressed together while the wax is hot. After the wax has cooled and formed an assembly comprising the gallium arsenide wafer and the submount, the assembly is measured to determine the uniformity of the wax layer forming the bond. If the wax layer varies more than 3 microns in thickness causing the gallium arsenide wafer to be mounted on an angle, the assembly will be reheated and pressure will be applied to even the wax layer. This procedure to achieve a uniform wax bond is cumbersome and time consuming.
A common processing step is metallization of a surface of the gallium wafer after the wafer has been thinned. Metallization of a surface of a semiconductor chip allows soldering or eutectic bonding to the surface for attaching the semiconductor chip to a heat sink or package. A preferred approach is metal sputtering which yields better metal coverage and better metal adhesion to the wafer than metal evaporation. Metal sputtering is performed at temperatures which liquify most commonly used waxes.
It would be a great advantage if a wafer mounting process could be developed which forms a strong bond, does not liquify at wafer process temperatures, will not bow or warp the gallium arsenide wafer significantly, and where the bonding material has a uniform thickness, yet is simple to apply and later remove in a production environment.